Low voltage indicator



0m. 27, 1959 s, P, KELLER 2,910,648

LOW VOLTAGE INDICATOR Filed Dec. 51, 1956 FIG. I

I NVENTOR 2 SEYMOUR P. KELLER AGENT United States Patent 2,910,648 LOW VOLTAGE INDICATOR Seymour P. Keller, Ponghkeepsie, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Application December 31, 1956, Serial No. 631,652

1 3 Claims. (Cl. 324-94) charge or electroluminescent devices but these devices require an appreciable signal to produce and sustain the condition which may be visually observed and serious circuit limitations are frequently encountered in providing a sutficient signal for these devices. Considerable additions to circuits have been necessary to provide the desired signal levels for the conventional visual indicator devices. A specific example of such circuit additions for a glow'discharge type of device is illustrated in Patent No. 2,772,410.

What has been discovered is that a low voltage indicator may be constructed wherein a setof ingredientsfmay enter into a chemical reaction that is reversible under the influence of an applied potential and which in one condition of the reaction ions or molecules having a distinctive color are present. Such an indicator is known as a chemical electrolytic cell and may be used to sense the presence of a small difference of potential, at a point in a circuit without the addition to the circuit of means necessary to drive a device having a large signal level.

A primary object of this invention is to provide a method of visually detecting the presence of aditference of potential.

Another object of this invention is to provide a low voltage indicating device. v

A related object is to provide an improved visual indicator of current flowing in a transistor.

Other objects of the invention will b' pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode, which has been contemplated, of applying that principle.

In the drawings:

Figure 1 is a schematic view-of an electrolytic cell wherein one reversible chemical reaction is illustrated as an example of the application of the technique of this invention.

Figure 2 is a schematic view of a chemical electrolytic cell wherein another reversible chemical reaction is illustrated.

In all chemical reactions there is a point of thermodynamic equilibrium which may be described as a condition of the reaction wherein the Gibbs free energy of the products of the reaction is equal to the Gibbs free energy of the reactants and it is toward this condition that all reactions tend to proceed. A discussion of the Gibbs free energy concept may be found in any standard text on thermodynamics such as Thermodynamics and the Free Energy of Chemical Substances, by G. N. Lewis and M. Randall, McGraw Hill, 1923. When a chemical reaction has attained equilibrium, there has been a decrease in the free energy of the system. If energy is then applied to and absorbed by the system in such a way that some of the products of the reaction can disappear and more of the reactants can appear, then the reaction is said to be reversible.

,If electrical energy is supplied to a reversible chemical reaction that has attained minimum free energy, the

reaction can be made to reverse such that the system is forced into a state of higher free energy. the reactants are colored, then these colored reactants, which may be ions or molecules, will appear when the system is forced'into this state of higher free energy. The appearance of this color would then constitute the manifestation of applied electrical energy and'would serve as an indicator of the presence of a potential difference.

A first example of such a reaction is shown schematically in Figure l. Referring now to Figure 1 a suitable container 1 is provided, the function of which is to re tain the ingredients of the reaction in contact with each other. A pair of inert electrodes such as platinum shown as elements 2 and 3 are placed in the container 1 and separated from each other. A source of potential which.

may be for example the difierence between a reference potential and a point in a transistor circuit at which it is desired to sense the flow of current is applied between electrodes 2 and 3 and is illustrated as element 4. A; means of producing a low impedance path between elec trodes 2 and 3 is illustrated as a switchS and is in-f eluded for purposes of accelerating the return of the reaction to the minimum free energy.

jilhe ingredientsfof a reversible chemical reaction are placed in the container 1 and the ingredients are so se le'cted that the reaction is reversible and will be responsive to the dlfierence of potential desired to be sensed and" that ions or molecules having a characteristic color, will become present as the reaction reaches a state of higherthan minimum free energy.

, first exampleof such a set of ingredients as illus trated inFigure 1 is the reversible reaction of mercury. (Hg) and iodine (I). p tion, written in the thermodynamic forward direction de-i The following chemical equascribes the performance of the ingredients.

a 8 S 2 2-l- The above Reaction 1 forms Hg l -H in its minimum energy'state and-it is toward this minimum state that the reaction tends to go when not under the influence of external forces. The-reaction is responsive to a potential of about 0.6 volt, which potential is effective in reversing the reaction and in causing I +Hg to be formed. The 1 ion has a brown color characteristic and will appear in thevicinity of the positive electrode which in Figure 1 has arbitrarily been designated to be electrode 2. The response time for this reaction is very rapid. It has been found to be almost instantaneous when a current of approximately one milliampere flows through the reaction between electrodes 2 and 3.

In the event that it is desired to return the reaction to the minimum energy state in less time than would be required for the reaction to normally return the electrodes 2 and 3 may be connected to each other through an appropriate low electrical impedance path. Such a path has been schematically shown in Figure l as a switch 5 capable of removing the applied potential and short circuiting electrodes 2 and 3. When switch 5 is set to short circuit electrodes 2 and 3 the brown characteristic color of the I ion disappears in about five seconds depending Patented Oct. 27, 1959* If some of on the actual physical quantity of the ingredients making up the reaction, and the physical makeup of the cell.

A second example of a reversible chemical reaction that is responsive to electrical potential and which has a distinctive colored ion present in its higher free; energy state is illustrated in Figure 2 wherein elements; serving the same function as in Figure l have been given the same reference numerals. The chemical, equation for the reaction of Figure 2 written in the thermodynamic forward direction is asfollows.

This Reaction 2 forms Fe+ and CNS- ions in the minimum energy state and is responsive to a potential of about 1.1 volts which causes the reaction to proceed to a state of higher energy. In the higher energy state Fe and Fe(CNS) are present and the Fe(CNS-) ion has. a characteristic red colorand it appears in the vicinity of the positive electrode. The response time for this reaction is about two seconds when a current of 200 to 400 microamperes flows between electrodes 2, and. 3 and when switch is set to short circuit electrodes 2 and 3 most of the characteristic color disappears in thirty' seconds.

In Reactions 1 and 2 it should be noted that the presence of Hg and Fe respectively augments the driving force of the reaction to the right. This can be. shown by simple application of the law of mass action. In the case of Figure 1, Hg is supplied by a pool of Hg in which the platinum electrode 3 is immersed. In the case of Figure 2, Fe is supplied in that electrode 3 is Fe. V

A. wide range of chemical reactions are suitable for use in the indicator of this invention, it being necessary only that the reaction be essentially complete in the thermodynamic forward direction at least to the point where there is no significant number of colored ions or molecules present, that there be an observable color change due to the appearance of colored ions or molecules in the reverse direction, that the reaction be reversible in that no gasses or precipitates be formed and that thereaction respond to the application of a potential by: assuming a higher free energy state. Many reactions otherwise suitable-for the indicator of this invention have no colored ionsor: molecules in the reverse direction. Such reactions can be rendered suitable by including as an ingredient of the reaction, an indicator whichcan impart a distinctive color when it reacts with a particular ion or molecule that is present in the higher free energy state. An examplewould be the use of the indicator phenolpthalein. If the products are neutral and the reactants are acidic or basic then the applied potential would result in the appearance of a pink or blue color when the reactants form- Other in dicators, sensitive to either pH change, or to appearance of certain specific ions or molecules are available in the art.

The driving potential for most suitable chemical re-.

actions lies between 0 and 4.0 volts and any particular reaction will have a unique voltage dependent upon con- 1 reactants; and the geometrical and optical layout of the physical structure of the reaction. The response time in the forward direction is a function primarily of ionic mobilities and this is inherently slow. If the electrodes are short circuited the forward reaction is increased and the thermodynamic stability is achieved more rapidly.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled. in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the following. claims.

What is claimed is:

1. A low voltage indicator comprising: a plurality of electrodes immersed in anelectrolytic. solution, signal in-- troduction means capable of applying. a difference of po tential between at least two of said electrodes, said electrolytic solution containing ingredients which take part in a reversible chemical reaction, said reaction being sensitive to a low electrical potential, having an inherent thermodynamic stability at a low free energy state, being capable-of being driven to av state of higher free energyby the application of a low electrical voltage, at least one of said ingredients being capable of imparting a characteristic color at said higher free energy state, and means for maintaining the ingredients of said reaction in; physical contact with each other and with said electrodes.

2. The low voltage indicator of claim 1 wherein said ingredients are Hg l and iodideion and. said reversible chemical reaction is 3. The low voltage indicatorof claim 1 wherein said ingredients are ferrous and thiocyanate. ions and said reversible chemical reaction is Fe+2Fe(0NS-) @3Fe+ IZCNS References. Cited in the file of this patent Hutchison July 29, 1913 

